Optimized configurations to integrate steering diodes in low capacitance transient voltage suppressor (tvs)

ABSTRACT

A transient-voltage suppressing (TVS) device disposed on a semiconductor substrate including a low-side steering diode, a high-side steering diode integrated with a main Zener diode for suppressing a transient voltage. The low-side steering diode and the high-side steering diode integrated with the Zener diode are disposed in the semiconductor substrate and each constituting a vertical PN junction as vertical diodes in the semiconductor substrate whereby reducing a lateral area occupied by the TVS device. In an exemplary embodiment, the high-side steering diode and the Zener diode are vertically overlapped with each other for further reducing lateral areas occupied by the TVS device.

This patent application is a Divisional application and claims thePriority date of a co-pending application Ser. No. 13/869,869 filed bythe same Applicants of this application on Apr. 24, 20013. ApplicationSer. No. 13/869,869 is a Divisional application of another applicationSer. No. 12/286,817 filed on Oct. 1, 2008 and now issued into U.S. Pat.No. 8,431,958. Application Ser. No. 12/286,817 is a Continuation in Part(CIP) application of a another patent application with a Ser. No.11/606,602 filed by a common Inventor of this application on Nov. 20,2006 and now issued into U.S. Pat. No. 7,880,223. This patentapplication is also a Continuation in Part (CIP) application of anotherapplication with a Ser. No. 11/600,696 filed by a common Inventor ofthis application on Nov. 16, 2006 now issued into U.S. Pat. No.7,781,826. The Disclosures made in the application Ser. Nos. 11/600,696,11/606,602, 12/ 286,817 and 13/869,869 are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to a circuit configuration and method ofmanufacture of a transient voltage suppressor (TVS). More particularly,this invention relates to an improved circuit configuration and methodof manufacture with optimized configuration to integrate steering diodesfor achieving a reduced capacitance for a transient voltage suppressor(TVS).

2. Description of the Relevant Art

The transient voltage suppressors (TVS) are commonly applied forprotecting integrated circuits from damages due to the inadvertentoccurrence of an over voltage imposed onto the integrated circuit. Anintegrated circuit is designed to operate over a normal range ofvoltages. However, in situations such as electrostatic discharge (ESD),electrical fast transients and lightning, an unexpected and anuncontrollable high voltage may accidentally strike onto the circuit.The TVS devices are required to serve the protection functions tocircumvent the damages that are likely to occur to the integratedcircuits when such over voltage conditions occur. As increasing numberof devices are implemented with the integrated circuits that arevulnerable to over voltage damages, demands for TVS protection are alsoincreased. Exemplary applications of TVS can be found in the USB powerand data line protection, Digital video interface, high speed Ethernet,Notebook computers, monitors and flat panel displays.

FIG. 1A-1 shows a conventional TVS circuit implemented with diode arraycommonly applied for electrostatic discharge (ESD) protection of highbandwidth data buses. The TVS array includes a main Zener diode operatedwith two sets of steering diodes, i.e., the high side steering diode andthe low side steering diode. The high side steering diode connects tothe voltage source Vcc and the low side steering diode connects to theground terminal GND with an input/output port connected between the highside and low side steering diodes. The Zener diode has a large size tofunction as an avalanche diode from the high voltage terminal, i.e.,terminal Vcc, to the ground voltage terminal, i.e., terminal Gnd. At atime when a positive voltage strikes on one of the I/O (input/output)terminal, the high side diodes provide a forward bias and are clamped bythe large Vcc-Gnd diodes, e.g., the Zener diode. The high side and lowside steering diodes are designed with a small size to reduce the I/Ocapacitance and thereby reducing the insertion loss in high-speed linessuch as fast Ethernet applications.

As an industry trend, the steering diodes are integrated with the Zenerdiodes. FIGS. 1A-2 and 1A-3 are two diagrams to show the integration ofthe steering diodes with the Zener diode. The high side and low sideterminals are not visible from the outside. FIG. 1A-2 shows theintegration of the high side steering diodes and low side steeringdiodes with a uni-directional Zener diode. To the outside, the diodeunit looks like a Zener diode with very low capacitance, but internallyhigh-side and low-side diodes are integrated with a Zener diode. Theinternal circuit is the same as the circuit in FIG. 1A-1. The I/Oterminal is the cathode, and the GND terminal is the anode, and the VCCterminal may be internalized so that it is not noticed from the outside.FIG. 1A-3 shows the integration of the high side steering diodes and lowside steering diodes with a bi-directional Zener diode circuit. However,for modern application to the electronic devices, the protection circuitimplemented with such integration must be carried out without increasingthe layout areas. Furthermore, there must have careful designoptimization to obtain the best tradeoff between the capacitance and theforward biased voltage of the steering diodes in order to achieve a goodoverall voltage clamping.

FIG. 1B shows a standard circuit diagram for a conventional TVS circuitand FIG. 1B-1 is a cross sectional view for showing the actualimplementation of the TVS circuit applying the CMOS processingtechnologies to provide the TVS circuit as integrated circuit (IC)chips. As shown in FIG. 1B-1, the device is manufactured using the CMOSprocessing technologies to produce diodes and NPN and PNP transistors inthe semiconductor substrate with the diodes and the transistors extendedalong a lateral direction. The TVS circuits produced by implementing thedevice layout and configurations thus occupy greater areas on asubstrate. It is therefore difficult to miniaturize the electronicdevice protected by TVS circuits as shown in FIG. 1B-1.

The inventor of this patent application disclosed a TVS circuit in apending patent application U.S. Ser. No. 11/606,602 with new andimproved device configuration shown in FIG. 1C. This application is aContinuation-in-Part (CIP) and claims the Priority of application Ser.No. 11/606,602. The disclosures made in patent application Ser. No.11/606,602 are hereby incorporated by reference in this patentapplication. FIG. 1C shows a TVS circuit implemented with a main Zenerdiode formed in a P Body/N-Epi junction. The TVS circuit as shown inFIG. 1C has significant improvement by reducing the areas occupied bythe device because the main Zener diode and high side diodes are nowformed with a vertical configuration. This circuit uses two I/0terminals and two corresponding sets of high side and low side diodesbut reversed the conductivity types of each region. The high sidesteering diodes is further insulated from the main Zener diode withisolation trenches thus prevent inadvertent turning on of the parasitictransistors along the lateral direction.

However, there are still further demand to reduce the areas occupied bythe high side and low side steering diodes. Also, there are additionalrequirements to further reduce the capacitance of the steering diodes.Therefore, there are demands to provide new and improved deviceconfigurations with new structural layout and manufacturing method toachieve these goals. The new device configuration and method ofmanufacturing must also take into consideration a process of designoptimization of the epitaxial layer with controlled doping concentrationand epitaxial thickness to obtain the best tradeoff between thecapacitance reduction and maintaining appropriate Zener breakdownvoltage.

Therefore, a need still exists in the fields of circuit design anddevice manufactures for providing a new and improved circuitconfiguration and manufacturing method to resolve the above-discusseddifficulties. Specifically, a need still exists to provide new andimproved TVS circuits that can provide low cost high-density TVScircuits with reduced capacitance and good voltage clamping performancefor portable electronic devices.

SUMMARY OF THE PRESENT INVENTION

It is therefore an aspect of the present invention to provide animproved TVS structural configurations implemented with high-side diode,low-side diode functioning as steering diodes. The steering diodes areintegrated with a main Zener diode wherein the high-side diode, thelow-side diode and the main Zener diode are all manufactured as verticaldiodes in a semiconductor substrate. The high-side diode now overlapswith the main Zener diode such that the TVS now occupies significantlyless area to allow for further miniaturization of the electronic devicesprotected by the improved TVS of this invention. The low-cost TVScircuits disclosed in this invention can achieve TVS protections withsmall silicon die footprint to overcome and resolve the above discussedlimitations and difficulties as encountered by the conventional deviceconfiguration and manufacturing methods for producing TVS devices.

Moreover, it is another aspect of the present invention to provide animproved TVS structural configurations implemented with a pair ofsteering diode comprising a high side steering diode and a low sidesteering diode. The steering diodes are integrated with a main Zenerdiode wherein the high-side diode, the low-side diode and the main Zenerdiode are all manufactured as vertical diodes in a semiconductorsubstrate. The vertical diode structures require smaller chip area ascompared to lateral structures because they have only one terminal onthe top surface, while the lateral device structures have both theterminals on the top surface.

Another aspect of this invention is that a configuration with a singleterminal on the top surface and another terminal on the bottom surfacefor each diode eliminates the undesirable effect of having bothterminals on the top surface. The vertical diodes as now disclosed isdifferent from the configuration that has both terminals formed on thetop surface thus restricting the current flow to the lateral directionfrom one terminal to another near the top surface resulting in a higherseries resistance. In contrast, the device of the present invention hasthe vertical currents transmitted between the top and the bottomterminals thus spreading the electric currents deep into thesemiconductor to provide better device performance because of lowerseries resistance and higher maximum current density.

Another aspect of this invention is to provide an improved TVSstructural configurations implemented with high-side diode, low-sidediode functioning as steering diodes for integrating with a main Zenerdiode in a semiconductor substrate. The high-side diode of thisinvention is able to achieve a significantly reduced junctioncapacitance by lightly doping an epitaxial layer between the N-typeburied layer (NBL) and a shallow P+ region, and achieving a bettercurrent spreading because of a vertical structure. The NBL is heavilydoped to further enhance the electric current to spread throughout theNBL.

Another aspect of this invention is to provide an improved TVSstructural configurations implemented with high-side diode, low-sidediode functioning as steering diodes for integrating with a main Zenerdiode in a semiconductor substrate. The high-side diode, the low-sidediode and the main Zener diode are all manufactured as vertical diodessuch that there is no surface current. The TVS devices disclosed in thisinvention therefore have improved ruggedness because as discussed abovethat the vertical diode structures of this invention have lower seriesresistance (resulting in reduced power dissipation) and improved currentspreading.

Another aspect of this invention is to provide an improved TVSstructural configurations implemented with high-side diode, low-sidediode functioning as steering diodes for integrating with a main Zenerdiode with a N+ doped buried layer (NBL) in a semiconductor substrate.The devices according to this configuration has improved performancebecause the highly doped NBL layer will suppress transistor actionthrough the parasitic vertical PNP transistor, which is part of aparasitic thyristor (PNPN) structure formed from one I/O to another I/O.A weaker PNP transistor will ensure that the parasitic thyristor doesnot turn on in applications that require the Vcc and Gnd terminals to beleft floating.

Another aspect of this invention is to provide an improved TVSstructural configurations implemented with high-side diode, low-sidediode functioning as steering diodes for integrating with a main Zenerdiode with a N+ doped buried layer (NBL) in a semiconductor substrate.The junction capacitance has a strong dependence on the dopingconcentration and decreasing the doping level by one order of magnitudereduces the junction capacitance by .about.70%.

Briefly in a preferred embodiment this invention discloses atransient-voltage suppressing (TVS) device disposed on a semiconductorsubstrate including a low-side steering diode, a high-side steeringdiode integrated with a main Zener diode for suppressing a transientvoltage. The low-side steering diode and the high-side steering diodeintegrated with the Zener diode are disposed in the semiconductorsubstrate and each constituting vertical diodes in the semiconductorsubstrate whereby reducing a lateral area occupied by the TVS device. Inan exemplary embodiment, the high-side steering diode and the Zenerdiode are vertically overlapped with each other for further reducinglateral areas occupied by the TVS device. In another exemplaryembodiment, the Zener diode further includes a buried source-dopedregion below a source region. In an exemplary embodiment, the high-sidesteering diode further includes a lightly doped body dopant epitaxiallayer disposed between a buried source dopant layer and a shallow bodydopant region having a higher body dopant concentration for to achievelow junction capacitance of the high-side steering diode. In anotherexemplary embodiment, a high voltage electrode is disposed on a topsurface of the semiconductor substrate and a low voltage electrode isdisposed on a bottom surface of the semiconductor substrate forconducting a current through the low-side steering diode and thehigh-side steering diode integrated with the Zener diode as verticaldiodes whereby surface current across a lateral direction of thesemiconductor is totally eliminated. In another exemplary embodiment,the deep buried source dopant region below the source dopant regioneliminating lateral latch up of turning on a parasitic bipolartransistor in the semiconductor substrate. In another exemplaryembodiment, the vertical diodes of the TVS device are disposed in anepitaxial layer having a light body dopant concentration for reducingcapacitances with a corresponding epitaxial layer thickness foroptimizing a breakdown voltage of the high-side and low-side steeringdiodes. In another exemplary embodiment, the buried source dopant regionhaving a maximum source dopant concentration achievable through anauto-doping without a diffusion process and satisfying a vertical Zenerbreakdown voltage requirement.

In another preferred embodiment, the present invention further disclosesan electronic device formed as an integrated circuit (IC) wherein theelectronic device further includes a transient voltage-suppressing (TVS)device for suppressing a transient voltage in the electronic device. Thetransient-voltage suppressing (TVS) device is disposed on asemiconductor substrate including a low-side steering diode, a high-sidesteering diode integrated with a main Zener diode for suppressing atransient voltage. The low-side steering diode and the high-sidesteering diode integrated with the Zener diode are disposed in thesemiconductor substrate and each constituting a vertical PN junction asvertical diodes in the semiconductor substrate whereby reducing alateral area occupied by the TVS device. In an exemplary embodiment, thehigh-side steering diode and the Zener diode are vertically overlappedwith each other for further reducing lateral areas occupied by the TVSdevice. In another exemplary embodiment, the Zener diode furtherincludes a buried source-doped region below a source region. In anexemplary embodiment, the high-side steering diode further includes alightly doped body dopant epitaxial layer disposed between a buriedsource dopant layer and a shallow body dopant region having a higherbody dopant concentration to achieve low junction capacitance of thehigh-side steering diode. In another exemplary embodiment, a highvoltage electrode is disposed on a top surface of the semiconductorsubstrate and a low voltage electrode is disposed on a bottom surface ofthe semiconductor substrate for conducting a current through thelow-side steering diode and the high-side steering diode integrated withthe Zener diode as vertical diodes whereby surface current across alateral direction of the semiconductor is totally eliminated. In anotherexemplary embodiment, the deep buried source dopant region below thesource dopant region eliminating lateral latch up of turning on aparasitic bipolar transistor in the semiconductor substrate. In anotherexemplary embodiment, the vertical diodes of the TVS device are disposedin an epitaxial layer having a light body dopant concentration forreducing capacitances with a corresponding epitaxial layer thickness foroptimizing a breakdown voltage of the high-side and low-side steeringdiodes. In another exemplary embodiment, the buried source dopant regionhaving a maximum source dopant concentration achievable through anauto-doping without a diffusion process and satisfying a vertical Zenerbreakdown voltage requirement.

The present invention further discloses a method for manufacturing anelectronic device with an integrated transient voltage suppressing (TVS)circuit. The method includes a step of applying a standard DMOSmanufacturing process to manufacture vertical PN junctions to functionas a low-side steering diode and a high-side steering diode integratedwith a vertical Zener diode for reducing lateral areas occupied by theTVS device.

These and other objects and advantages of the present invention will nodoubt become obvious to those of ordinary skill in the art after havingread the following detailed description of the preferred embodiment,which is illustrated in the various drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A-1 shows the circuit of a conventional TVS circuit implementedwith diode array commonly applied for electrostatic discharge (ESD)protection.

FIGS. 1A-2 and 1A-3 are two diagrams to show the integration of thesteering diodes with the Zener diode to achieve low capacitance inunidirectional and bi-directional blocking TVS diodes respectively.

FIG. 1B shows a standard circuit diagram for a conventional TVS circuitand FIG. 1B-1 is a cross sectional view for showing the actualimplementation of the TVS circuit applying the CMOS processingtechnologies to provide the TVS circuit as integrated circuit (IC)chips.

FIG. 1C shows a TVS circuit implemented with diodes formed as verticaldiodes to reduce the size of the TVS circuit.

FIGS. 2 to 4 are cross sectional views for the integrated Zener diodewith the high side and low side steering diodes illustrated withequivalent circuits of TVS devices implemented with N+ buried layer andisolation trenches to form vertical TVS diode arrays of this inventionto reduce the areas occupied by the diode array.

FIGS. 5A to 5B are top views of the layout of the TVS devices to showthe reduced areas required by implementing the vertical diode array ofthis invention.

FIG. 6 is a cross sectional view for illustrating the capacitancecomponents of a TVS circuit configured with N-buried layer (NBL) TVSZener.

FIG. 7 is a diagram for illustrating the low capacitance designs for thesteering diode as implemented for optimizing the design parameters inthis invention.

FIG. 8 is a diagram for showing the variation of the junctioncapacitance versus the doping concentration N.sub.D for an abrupt N+-Pjunction.

FIG. 9 is a diagram for showing the variation of the depletion widthW.sub.D versus the doping concentration N.sub.D for an abrupt N+-Pjunction.

FIGS. 10A-10D are cross sectional views illustrating the forming of theNBL and the trigger implant layer.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Refer to FIG. 2 for a side cross sectional view illustrated withequivalent circuit of a transient voltage suppressor (TVS) 100 of thisinvention. The TVS 100 is formed on a heavily doped P+ semiconductorsubstrate 105 which supports a P two-layer epitaxial layer 110 thatincludes a bottom P-epitaxial 110-1 and a top P-epitaxial layer 110-2with a backside metal 101 disposed below the bottom surface to functionas a ground terminal. The TVS 100 includes a P+ region high-sidesteering diode and Zener diode overlapping zone with a deep voltagebreakdown (VBD) trigger implant layer 115 implanted with P+ dopant ionsdisposed between the bottom epitaxial layer 110-1 and a N+ buried layer120 disposed below a top N+ source region 125. The Zener diode is formedfrom the buried layer 120 to the bottom epitaxial layer 110-1. A shallowP+ implant region 130 is formed near the top surface of the topP-epitaxial layer 110-2 to enhance the electrical contact with an I/Ometal pad 135. An oxide insulation layer 145 covering the top surfacehas openings to allow a Vcc pad 140 to contact the N+ source regions 125above the high-side diode and Zener diode overlapping zone and an I/Opad 135′ contacting the source region 125′ of the low side diode shownon the right side of the TVS 100, and for the I/O pad 135 to contact theshallow P+-implant region 130. The I/O pad 135 and the I/O pad 135′ maybe connected in the third dimension. The N+ source regions 125 has a gapin which the high-side diode is located from the top P-epitaxial layer110-2 to the N+ buried layer 120. The low side diode is located from thesource region 125′ to the two-layer epitaxial layer 110. The TVS 100further includes isolation trenches 150 to isolate the low-side steeringdiode with the high-side diode integrated with the overlapping Zenerdiode. A parasitic vertical PNP transistor is exists, from the shallowP+ implant region and the portions of the P-epitaxial region 110-2 belowit, to the N+ buried layer 120, to N-epitaxial layer 110-1 below. Byhaving a highly doped N+ buried layer 120, the transistor action isavoided. The parasitic vertical PNP transistor is part of a parasiticPNPN thysistor formed in the semiconductor regions between I/O metalpads 135 and 135′. A weaker PNP transistor will ensure that theparasitic thyristor does not turn on in applications that require theVcc and Gnd terminals to be left floating. It is desired to not allowthe steering diodes to breakdown, so the breakdown voltage of the Zenerdiode is made to be much lower than that of the steering diodes. The VBDtrigger layer sets the breakdown voltage of the Zener diode at a desiredlow value.

FIG. 3 is a cross sectional view for showing an alternate TVS 100′ ofthis invention. The TVS 100′ has a similar configuration as the TVS 100shown in FIG. 2 except that the deep voltage breakdown (VBD) triggerlayer 115′ is formed with a patterned implant to form a gap under thehigh side steering diode in order to avoid a high doping layer directlyunder the high-side steering diode. This may avoid inadvertently raisingthe doping concentration of the portion of P-epitaxial layer 110-2beneath the P+ implant region 130. This region should be kept at a lowdoping concentration to achieve low capacitance as explained below. FIG.4 is a cross sectional view for showing another alternate TVS 100″ ofthis invention. The TVS 100″ has a similar configuration as the TVS 100and 100′ shown in FIGS. 2 and 3 respectively except that the N+ buriedlayer 120′ is patterned with a gap in it and the deep voltage breakdowntrigger layer 115″ is formed adjacent and in between instead of underthe N+ buried layer 120′.

FIGS. 5A and 5B are top views for showing the layouts of a TVS accordingto a configuration shown in FIGS. 1B-1 and 2 respectively. As shown inFIG. 5A, the main Zener diode is formed on a separate area from the highside diode. In comparison, in FIG. 5B, the high side diode is overlappedwith the Zener diode and therefore the TVS is formed with much reducedarea compared with the TVS as that shown in FIG. 5A.

FIG. 6 is a cross sectional view shown with capacitance equivalentcircuit to illustrate the total capacitance of the Zener diode Cz incombination with the high-side and the low-side diodes C.sub.HS andC.sub.LS respectively. Assuming that C.sub.Z is much greater thanC.sub.HS or C.sub.LS, the total capacitance C.sub.Total can be expressedas:

CTotal=α_(PNP)*(C _(HS))+C _(LS) +C _((Pad))

Where α_(PNP) is the emitter to collector gain of the vertical PNPtransistor formed by P-epitaxial layer 110-2, N+ buried layer 120 andP-epitaxial layer 110-1, and C (Pad) is the pad capacitance. Accordingto the above equation, it is necessary to reduce the capacitance of thehigh-side and low-side steering diodes C_(HS) and C_(LS) in order toachieve a low capacitance for the TVS; since Cz is much greater than andin series with CHs, Cz has a negligible effect on CTotal. FIG. 7illustrates the depletion width WD for an abrupt N+/P-junction. For avertical diode the depletion width is in the vertical direction, so thedepth of the P-layer should be at least as large as the depletion widthWD. However, the P-layer depth should not be much larger than WD or itwill needlessly increase the forward resistance of the diode. For anabrupt N+ and P-junction, the junction capacitance Cj and breakdownvoltage V_(BD) are:

C_(j) α(N_(A))^(1/2)

V_(BD)□α□(N_(A) ^(−3/4)·(NPT)

Where N_(A) represents the doping concentration of the P-region and NPTrepresents the Non-Punch Through breakdown voltage. The capacitance ofthe steering diodes decreases with a higher breakdown voltage when thedopant concentration is reduced as that shown in FIG. 8 for showing thejunction capacitance Cj as a function of the dopant concentration andFIG. 9 for showing the depletion width W_(D) in the epitaxial layer asfunction of the dopant concentration. FIG. 8 shows the junctioncapacitance Cj rising with the P dopant concentration. Therefore,optimal performance of the TVS is achievable by determining a lowerepitaxial layer dopant concentration for the P-epitaxial layers 110-2and then using that dopant concentration to determine an optimalthickness of the P-epitaxial layer 110-2 according to a width of thedepletion layer thickness as shown in FIG. 9. For the high side diode,the capacitance is formed between the P+ implant region 130 and the NBL120, so the vertical distance of the region of P-epitaxial layer 110-2between P+ implant region 130 and N+ buried layer 120 should match thedepletion width to achieve low capacitance. That vertical distanceshould be kept close to the depletion width to avoid needlesslyincreasing the forward resistance of the diode. For the low side diode,the vertical distance from source region 125′ to substrate 105 shouldapproximately match the depletion width (taking into account the dopingconcentrations of epitaxial layers 110-1 and 110-2). The thickness ofthe first epitaxial layer 110-1 should also take into account both thedepletion width of the low side diode and also the distance from thehigh side diode; if the substrate 105 is too close to the high sidediode, some of the dopants from the substrate 105 may diffuse into theregion of the second epitaxial layer 110-2 under the contact implant 130and increase the doping there and thus increase the capacitance of thehigh side diode. In a preferred embodiment, the dopant concentration ofthe P-epitaxial layers 110-1 and 110-2 will be as kept low as possible,to ensure a low capacitance in the steering diodes. The N+ buried layer120 under the source region 125 is implanted with a highest dose withminimum diffusion by applying an automatic doping process whilesatisfying the breakdown voltage requirements of the vertical Zenerdiode.

FIGS. 10A-D demonstrate a method for forming the NBL for a devicesimilar to device 100′ in FIG. 3. FIG. 10A shows a heavily doped P+substrate 105 with a lightly doped first P-epi layer 110-1 grown overit. In FIG. 10B, a masked implant (mask not shown) is performed to formthe N+ implant region 121. In FIG. 10C, a drive-in is performed todiffuse the N+ implant region 121 to form NBL 120. In FIG. 10C, anothermasked implant (mask not shown) is performed to form the P+ VBD triggerimplant layer 115′ underneath the NBL 120. In FIG. 10D, the second P-epilayer 110-2 is grown over the first P-epi layer 110-1. The NBL 120 maydiffuse slightly into the second epitaxial layer 110-2.

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is not to be interpreted as limiting. For example, theconductivity types of the semiconductor regions could be reversed sothat the P-type regions are now N-type regions and vice versa. In thiscase the high side diode and the low side diode would swap positions;also the topside of semiconductor would have the lower voltage and thebottom side would have the higher voltage. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter reading the above disclosure. Accordingly, it is intended that theappended claims be interpreted as covering all alterations andmodifications as fall within the true spirit and scope of the invention.

We claim:
 1. A method for manufacturing a transient voltage suppressing(TVS) device: forming a pair of vertical steering diodes integrated witha vertical Zener diode for reducing lateral areas occupied by the TVSdevice wherein the pair of steering diodes comprises a high sidesteering diode and a low side steering diode.
 2. The method of claim 1wherein: said step of forming said pair of steering diodes integratedwith said main Zener diode further comprising a step of forming thefirst of the pair of steering diodes and the Zener diode verticallyoverlapping with each other for further reducing lateral areas occupiedby the TVS device
 3. The method of claim 2 further comprising a step of:forming a buried source-dopant region below a source region for saidZener diode.
 4. The method of claim 3 wherein: the forming said first ofthe pair of steering diodes further includes a step of forming a lightlydoped body dopant epitaxial layer between the buried source dopant layerand a shallow body dopant region with a higher body dopant concentrationfor achieving low capacitance and good contact for the first of the pairof steering diodes.
 5. The method of claim 3 further comprising: formingtwo epitaxial layers over the semiconductor substrate, wherein theburied source-dopant region is located at the junction of the twoepitaxial layers.
 6. The method of claim 3 wherein: said step of formingsaid buried source dopant region below the source dopant region furtherincluding heavily doping the buried source dopant region for eliminatinga turning on a parasitic bipolar transistor in the semiconductorsubstrate.
 7. The method of claim 3 further comprising a step of:disposing said vertical diodes of the TVS device in an epitaxial layerhaving a light body dopant concentration for reducing capacitances witha corresponding epitaxial layer thickness for optimizing a forwardresistance and junction capacitance of the high side and low sidesteering diodes.
 8. The method of claim 3 wherein: the source dopant isN-type, the first of the pair of steering diodes is the high sidesteering diode, and the second of the pair of steering diodes is the lowside steering diode.
 9. The method of claim 2 further comprising:forming at least an isolation trench in said semiconductor substrate forseparating the second of the pair of steering diodes from the first ofthe pair of steering diodes overlapping with the Zener diode.
 10. Themethod of claim 3 wherein: forming a buried source dopant region furthercomprising growing a first epitaxial layer on said semiconductorsubstrate, performing mask implant on said first epitaxial layer withsource dopants and growing a second epitaxial layer on said firstepitaxial layer.
 11. The method of claim 3 further comprising:optimizing the high side and low side steering diodes by selecting a lowdopant concentration of an epitaxial layer for achieving a low junctioncapacitance and selecting a thickness of said epitaxial layer forsatisfying a depletion width of said high side and low side steeringdiodes.
 12. The method of claim 11 wherein: said step of optimizing saidhigh side and low side steering diodes of said TVS further comprising astep of selecting said thickness of said epitaxial layer to avoidincreasing a forward resistance of said high side and low side steeringdiodes.